The present invention relates to a torque distribution control apparatus for a four wheel drive.
In general, there has been known a torque distribution control apparatus for a four wheel drive which drives four wheels by engine outputs, wherein the torque distribution for each wheel is not always equivalent but is variably controlled so as to become optimum correspondingly to operating conditions.
Japanese Unexamined Patent Publication No. 1-247223 has disclosed that the turning movement of the car is classified under three parts at the time of turning start, during constant turning and at the time of turning escape, so that torque distribution control is executed correspondingly to the turning state of the car on the basis of a steering angle and a change rate thereof for front wheels. In other words, the brake absorbs the driving force of the front wheels to decrease the torque distribution for the front wheels such that the small turn of the car can be enhanced at the time of turning start. On the other hand, the brake absorbs the driving force of the rear wheels to decrease the torque distribution for the rear wheels such that the straightness can be enhanced at the time of turning escape. Consequently, the burden of the front and rear tires can become uniform. The torque distribution is uniformly executed for the front and rear wheels in the state of straight running.
The car moves unsteadily owing to a great side wind at the time of straight running. Also in the case where the car moves straight by virtue of excessive driving force, the slip may easily be caused on the wheels so as to move unsteadily. When the car moves unsteadily, a driver corrects steering. In that case, the steering angle is changed. Consequently, it is decided that the car is in the state of turning start. Accordingly, there is executed the torque distribution control at the time of turning correspondingly to the steering state.
When the torque distribution control at the time of turning start is executed against unsteady movement, the torque distribution for the rear wheels is increased. Consequently, the slip of the rear wheels is increased so that the movement of the car tends to be undesirably changed. In the case where a friction coefficient (hereinafter referred to as .mu. if necessary) of a road is low, the above-mentioned tendency is made remarkable.